Adaptive multi‐degree‐of‐freedom in situ bioprinting robot for hair‐follicle‐inclusive skin repair: A preliminary study conducted in mice

Abstract Skin acts as an essential barrier, protecting organisms from their environment. For skin trauma caused by accidental injuries, rapid healing, personalization, and functionality are vital requirements in clinical, which are the bottlenecks hindering the translation of skin repair from benchside to bedside. Herein, we described a novel design and a proof‐of‐concept demonstration of an adaptive bioprinting robot to proceed rapid in situ bioprinting on a full‐thickness excisional wound in mice. The three‐dimensional (3D) scanning and closed‐loop visual system integrated in the robot and the multi‐degree‐of‐freedom mechanism provide immediate, precise, and complete wound coverage through stereotactic bioprinting, which hits the key requirements of rapid‐healing and personalization in skin repair. Combined with the robot, epidermal stem cells and skin‐derived precursors isolated from neonatal mice mixed with Matrigel were directly printed into the injured area to replicate the skin structure. Excisional wounds after bioprinting showed complete wound healing and functional skin tissue regeneration that closely resembling native skin, including epidermis, dermis, blood vessels, hair follicles and sebaceous glands etc. This study provides an effective strategy for skin repair through the combination of the novel robot and a bioactive bioink, and has a promising clinical translational potential for further applications.

severe infection. 3,4 However, the self-repairing ability of skin is limited and wounds larger than 4 cm in diameter do not heal well without intervention, 5 leading to numerous studies seeking to develop improved skin wound repair techniques.
The requirements for skin wound repair are unique, including (i) rapid healing, (ii) personalization, and (iii) functionality. The first point relates to the requirement for skin wounds to be treated rapidly following injury to prevent fluid loss or infection. Second, the irregular nature of skin wounds, which vary in morphological characteristics and location, necessitates personalized treatment. Finally, the regenerated skin must have appendant organs that support normal physiological functions.
Current skin wound repair methods include autologous skin transplantation, artificial skin substitutes, and three-dimensional (3D) bioprinting. [6][7][8][9] Among these, covering the excised fullthickness wound with autologous skin grafts is considered the "gold standard" for treating severe wounds because skin grafts offer versatility and the capacity for self-regeneration. However, the applicability of autologous skin grafts remains restricted by the limited supply of available donor sites, 10 making it difficult to reconstruct the skin damaged by large wounds. Artificial skin substitutes can act as wound coverage and be used as a space filler to create a vital protective layer in cases of severe tissue loss and facilitate the recovery of damaged tissues. Nevertheless, owing to the lack of necessary skin appendages such as hair follicles, the newly formed skin lacks the functionality of normal skin. In addition, the standardized production of artificial skin substitutes means that patient-specific requirements cannot be satisfied. 11 The 3D bioprinting technique has the capacity to deliver bioink to specific target sites on a layer-by-layer basis to construct tissues or organs capable of performing biological functions and activities. As such, it has been applied in numerous applications. 12 A new branch of bioprinting, in situ bioprinting, was proposed in 2007, and refers to the direct printing of bioink at a defect site to create or repair living tissues. 13 Since its inception, in situ bioprinting has shown great progress in the repair of superficial and internal tissues. [14][15][16] According to previous studies, 17,18 existing setups for in situ bioprinting are typically based on an open-loop three-axis motion platform, which implies that the printer can only perform calibratethen-print operations and is only applicable to static target surfaces. Recently, Zhu et al. described a closed-loop system to perform in situ 3D printing on a moving hand and deformable lung to compensate the movement of the printed target, 19,20 but this compensation system was based on a three-degree-of-freedom (three-DoF) printer which cannot satisfy the stereotactic bioprinting of skin repair due to the lack of DoF Furthermore, no bioprinting approach can yet fully replicate the morphological, biochemical and physiological properties of native skin. 17,[21][22][23][24][25] Most of them constructed 3Dbioprinted skin through vividly mimic the layered structure consisted of epidermis and dermis but ignored the necessary appendages important for physiological functions, and the incorporation of additional cell types and the patterning of more representative extracellular matrix (ECM) components is necessary.
Herein, we describe the development of an adaptive multi-DoF in situ bioprinting robot comprising a scanning system, a binocular visual system, and a six-DoF manipulator to process in situ bioprinting with epidermal stem cells (Epi-SCs), skin-derived precursors (SKPs) extracted from neonatal mice and Matrigel as bioink for skin repair. The conceptual design for the method is illustrated in Figure 1. For the needs of personalization, the topographical information of the skin wound was acquired via a structured light scanner to plan the printing path. Wound positions were identified through a binocular camera and fed back to the motion controller of the manipulator, which constituted a closedloop feedback system for real-time wound tracking in order to avoid printing errors due to unintended movement. The six-DoF manipulator was utilized for prompt cure and rapid healing, except for its portability, the three additional DoF ensured that the print head adjust the printing direction adaptively according to the topography of the skin wounds, facilitating its application to complicated surfaces. Epi-SCs, as well as dermal stem cells mixed with Matrigel, are used as bioink to pursue functionality requirement owing to their excellent capacity for promoting skin repair and the formation of skin appendages. The aim of the proposed robot is to establish an autonomous workflow for functional skin wound repair that requires minimal human intervention. Our approach advances conventional bioprinting in four aspects. First, integrated 3D scanning is able to acquire quantizable morphology of skin wounds, and adjust printing strategy based on actual situation, thereby achieving personalized skin treatment. Second, the proposed visual system can accommodate six-DoF wound motion to realize adaptive in situ bioprinting. Third, the manipulator increases the workspace and the ability to print on sophisticated surfaces, which enhances the adaptability for skin repair. Finally, the bioactive bioink confers physiological functionality to the regenerated skin. An in vivo study was conducted in mice to verify the practicability of the proposed bioprinting robot for the repair of functional skin, and to evaluate its application potential in clinical environments.

| 3D scanning and conformal path planning
A structured-light 3D scanner (Thunk3D) with submillimeter-level accuracy and resolution was used to acquire the point cloud data of the skin wounds. The point cloud data were then processed using Geomagic Studio software, and the deficient skin model can be acquired through Boolean operations. To fill the skin wounds, the volume of each wound was calculated in order to allocate the bioink appropriately. For small wounds, bioink was delivered directly via a point-to-point approach, whereas for large wounds, a planar toolpath was designed using Cura3D software (Ultimaker) and projected onto the reconstructed model to generate the conformal toolpath, which was divided into i points as a preset path to control the manipulator, with each point containing information in six coordinates. In addition to the three spatial coordinates, the other three coordinates denote the Euler angle of the point to guide the manipulator as it moves along the normal direction of the toolpath during in situ bioprinting.

| Spatial coordinate calculation based on binocular visual identification
Visual identification was performed using a color filter to account for the distinct coloration of skin wounds. Owing to reasons such as glare and shadow, among others, the range of pixel intensity values may vary for different images. A linear normalization procedure was performed to eliminate these effects. During segmentation, the images captured by the binocular camera were converted from the RGB to the HSV color space, making it possible to filter specific colors by adjusting hue, saturation, and value (i.e., brightness). A Gaussian blurring filter with a standard deviation of σ = 0.5 was used for image denoising, allowing the image gradients to be obtained before unwanted margins were removed via thresholding. 26

| Evaluation of in situ bioprinting conducted in mice
The BALB/c nu/nu mice (4-5 weeks old) were anesthetized with sodium pentobarbital (50 mg/kg). Symmetrical full-thickness skin wounds were created on the back of the mice using the skin biopsy punch with diameter of 2/5/10 mm, as described previously. 30

| Hematoxylin and eosin staining
Freshly regenerated skin samples were obtained from mice, and then fixed in 10% formalin or other fixatives for 12-24 h at RT. After dehydration, the tissues were embedded in paraffin for tissue section, and the section slices were rehydrated with 100% ethanol, 95% ethanol, 75% ethanol, and deionized water for 3 min each. Hematoxylin and eosin (H&E) were used to stain the nucleus and cytoplasm respectively, and the prepared samples were observed using a phasecontrast microscope (Nikon, Eclipse Ci-S). 32

| Cell proliferation
On culture days 0 (60 min after bioprinting process), 3, and 7, printed constructs were incubated with a mixture of culture medium and cell counting kit-8 (CCK-8) (Yeasen) at a volume ratio of 10:1 to investigate cell proliferation. After 1 h incubation at 37 C, the absorbance of formazan dye was determined at 450 nm using a Microplate Reader (Thermo Fisher Scientific). The obtained data was normalized to the cell number according to the pre-established standard curve. Three samples were tested in each time point.

| Flow cytometry analysis
The Epi-SCs were analyzed through flow cytometry, as described previously. 33 Briefly, the cells were suspended in PBS containing 1% bovine serum albumin at a density of 10 6 cells/ml. Cell aliquots was used to analyze 10,000 events using Cell Quest software.

| RNA isolation and real-time polymerase chain reaction
Total RNA was extracted using TRIzol (Takara) in accordance with the instructions provided by the manufacturer. First-strand cDNA was prepared using the PrimerScript™ RT Reagent Kit with gDNA Eraser (Takara) and oligo(dT) primers, and then stored at À20 C. Real-time polymerase chain reactions (PCR) were performed using SYBR ® Green on an analytik Jena qTOWER 3G system. As an internal control, the levels of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were quantified in parallel with the target genes. Normalization and fold changes were calculated using the ΔΔCt method. The primers used for murine gene amplification were shown in Table 1.

| Statistical analysis
All experiments were repeated at least three times, with the results expressed as the mean ± standard deviation (SD) unless stated otherwise. Statistical comparisons between two groups were evaluated using the Student's t-test. Statistical significance was set at p < 0.05.
where l i represents the calculated distance between the object and the camera and l 0i denotes the real distance. Furthermore, dP i denotes the recognized error for the object and θ represents the depression angle.
After eliminating the intrinsic parameters, the range errors were basically consistent for each position and angular deviations were affected by the depression angle to a substantially reduced extent, which indicates that our visual recognition algorithm can satisfy wound identification requirements with high accuracy.
In a rare study involving visual-based bioprinting, 19,20 researchers mostly chose an eye-to-hand framework, which means that the camera observes the robot within its workspace. 34 The eye-to-hand architecture is a relatively simple and robust framework, but suffers from several problems: (i) camera installation may interfere with the robot's workspace; (ii) movement of the manipulator may block the camera during tracking; and (iii) an appropriate trade-off between the workspace and tracking accuracies cannot be achieved. 35 In this study, an eye-in-hand structure was used, whereby the camera was mounted in a fixed position on the robot's end-effector. This is the first time that this structure was applied in robot-assisted in situ bioprinting. This In the eye-in-hand configuration, the manipulator was controlled by the camera through solving the relation between the camera coordinate system and the end-effector coordinate system. Here, the eyein-hand system obeys the following identity relation: where w e H i represents the homogeneous transformation matrix of the end-effector coordinate system relative to the base coordinate system of the manipulator. In addition, e c H denotes the homogeneous transformation matrix of the camera coordinate system relative to the endeffector coordinate system, which is the solution of Equation (3).
Next, w g H is the homogeneous transformation matrix of the calibration grid coordinate system relative to the base coordinate system of the manipulator, which is constant. Finally, g c H i is the homogeneous transformation matrix of the camera coordinate system relative to the calibration grid coordinate system. The subscript i varies and represents different samples. direction. This is illustrated in Figure 2h, where the movement before and after the compensation are indicated by the red and black arrows, respectively, proving that direction tracking is maintained during rotation.

| Control and evaluation of bioprinting on complicated surfaces
The bioprinting manipulator uses a series mechanism with six-DoF, and its kinetic model can be established using the Denavit-Hartenberg (D-H) method. 36 The coordinate systems were set at each joint of the manipulator (Figure 3a), with the corresponding transformation relationship expressed as  Table S1.
The kinematic model of the manipulator can be obtained by multiplying the transformation matrix of the six joints in turn to realize manipulator control: where 0 6 T represents the transformation from joint 0 to joint 6 and P denotes the position of the end of the manipulator, with R and p representing the orientation and spatial coordinates of the endeffector, respectively.
The workspace of the manipulator was computed by mechanism analysis and the Monte Carlo method separately in MATLAB 2018 (MathWorks). 37 Error accumulation is an inherent defect in serial manipulator, 38 rendering it necessary to evaluate the kinematic error.
The manipulator moves inside the entire workspace with a 5 mm pitch and the end-effector was tracked by a laser to calculate the kinematic error, which was represented in the workspace (Figure 3b this novel skin repair method, we attached freshly collected pigskin to a cube and printed the letters "THU" on three different composite surfaces simultaneously (Figure 3h; Movie S4), thereby further confirming the superiority of this innovative method.

| Cell culture and characterization between robotic bioprinting and manual implantation
In the selection of seed cells, adult cells, especially keratinocytes and fibroblasts, were chosen in most researches. 21,25,41 Amniotic fluidderived stem cells (AFSCs) were also a favorable choice in skin repair for their multipotent, as conducted in Skardal's research. 17 In consideration of the functional activities of regenerated skin tissue, especially necessary appendages reconstruction, chamber assay was used for reference. 42,43 It is the representative method for reconstructing the hair follicle structure, and has revealed that the reconstitution of appendages from disassociated cells requires epithelial-mesenchymal interactions. 44 And several studies have demonstrated the potential of progenitor stem cells to make hair follicles structure in skin substitutes. 45,46 Based on this, we applied Epi-SCs and dermal stem cells collected from neonate mouse as epithelial cell source and mesenchymal cell source.
To verify the characterization of the primary stem cells isolated from neonatal mice and to explore the impact of printing process on the cells, the cells were cultured in a Petri dish for 7 days to evaluate cell viability and proliferation after robotic bioprinting and hand implantation. The cells grew well in culture as the viabilities of Epi-SCs and SKPs were always maintained at approximately 90% in 7 days, and there was no significant difference (p > 0.05) between bioprinting and hand implantation (Figure 4a,b). In terms of cell proliferation, the Epi-SCs grew with adherence to the plate, whereas the SKPs grew in small aggregates and were suspended in the culture. This led to a gradual upward trend in the number of Epi-SCs over 7 days. SKPs proliferated significantly in the first 3 days, and slowed down in the next few days, which might owe to limitations in the nutrition interchanges caused by aggregation. But both cell types showed significant proliferation during the 7-day's culture (Figure 4c 49 were also detected by immunofluorescent staining, which showed no significant differences between the SKP-P and SKP-H cells, but the expression of the three markers increased compared to SKP-P0 (Figure 4h-j). The Epi-SCs grew and spread strongly in the Petri dishes without significant morphological differences being observed between the normal culture (Epi-P0), bioprinting (Epi-P), and hand implantation (Epi-H). Moreover, the expression of typical marker CD49f 50 increased in the printed and hand-implanted groups compared with the normal culture group (Figure 4k). Flow cytometry confirmed that CD29 and CD49f were expressed strongly and positively in all groups, while the expressions of Epi-P and Epi-H were higher than Epi-P0 (Figure 4e). Stem cells are fragile and sensitive to mechanical disturbance. Maintaining high levels of cell stemness is one of the obstacles to apply stem cells into clinical trials. 51 To investigate the influence of the bioprinting process on SKP stemness, the pluripotent genes Oct4, Sox2, Nanog, and c-Myc were detected via real-time PCR analysis after culturing for 3 days. The results showed that pluripotency was maintained both in the printed and hand-implanted groups, further implying that our bioprinting robot has little influence on cell stemness (Figure 4k). Additionally, a series of typical genes involved in the hair induction process of SKPs were detected. 47 The results showed that both the bioprinting and hand-implanted groups had a high expression of these functional genes, suggesting that the SKPs maintained a high potential to support hair follicle regeneration after the bioprinting process ( Figure 4f). These results indicate that these skin-derived stem cells possess the capacity to achieve in vivo skin repair with appendages, and the proposed bioprinting robot will not impede the functional expression of stem cells. process, the robot automatically recognized and tracked skin wounds when unintended motions, for example, breathing and twitching, occurred. The quantitative Epi-SCs, SKPs, and Matrigel mixture were prepared according to Method Section and loaded to the bioprinting robot. The bioink was in situ printed into the excisional wounds in accordance with the preset path (Figure 5a; Movie S5), with photographs captured before and after bioprinting ( Figure S4). In the 4-weeks' feeding, the mortality rate was 0% and no wound infection of major skin irritation was noted on any mouse. After 4 weeks, the newly regenerated hair-inclusive skin tissues were photographed for all wounds under a dissecting microscope, and the relevant hair shafts were counted (Figure 5b,d). The average number of hair shafts increased as the wound area increased in the bioprinting group, with the same trend observed in the hand-implanted group. Bioprinting promoted slightly greater hair follicle growth than observed in the control group, but the difference was not significant (Figure 5f). The regenerated skin tissues underwent H&E staining, revealing densely populated hair follicles and sebaceous glands (Figure 5c), and the latter were further confirmed by immunofluorescence (IF) staining with anti-Biotin, which identifies sebaceous glands specifically ( Figure 5g). 28 In addition, the presence of newly formed blood vessels in the regenerated skin was confirmed by IF staining with CD31, a marker typically expressed in endothelial cells to detect angiogenesis  54 To evaluate the durability of the neogenic hair shafts and the biocompatibility of the bioink, partial printed-treatment mice were fed for another 10 months after bioprinting for further observation. The results indicated that a substantial proportion of neogenic hair shafts remained after 10 months, with only a few shed. Moreover, H&E staining of the 10-month regenerated skin tissue indicated that hair follicles remained even after the hair shafts fell out. These results revealed that regenerated skin tissues through our proposed method were permanent in structure and functions, which were close to native healthy skin and showed no signs of teratoma formation ( Figure S5). As such, this study demonstrates that the combination of robotic in situ bioprinting and the bioactive bioink presents an effective opportunity for functional skin repair and has a promising clinical translational potential for rapid and accurate skin repair. Taking skin repair for instance, conventional autologous skin grafts and artificial skin substitutes are not able to satisfy the largearea skin treatments and personalization requirements for limited sources or rigid fabrication procedure. 10,11 Complex structure of skin, containing layered epidermis, dermis and important appendages with intricate spatial arrangements, renders the 3D bioprinting of the full skin organ challenging.

|
Aiming at these challenges, we proposed a novel strategy for skin wound repair to hit the key requirements of prompt cure, personalization and functionality. This method comprises an adaptive multi-DoF in situ bioprinting robot and functional bioink supporting appendagesinclusive skin repair. Herein, the concept of bioprinting robot integrating 3D scanning and closed-loop visual system as well as a manipulator was proposed for the first time to provide rapid on-site management of full-thickness wounds. The bioink was deposited quantitatively and precisely avoiding errors caused by irregular wounds and unexpected motions, which achieved accurate skin repair macroscopically. In conventional three-axis bioprinting process, the nozzle is constrained in the 2D plane along the direction of gravity, making it inevitable to generate stair-step effect between neighboring layers, which causes surface distortion of the structure. 66 In addition, curved surface of skin requires the nozzle to adaptively operate in the normal direction according to the specific surface shape, which is insufficient in conventional bioprinter. Under the need of fidelity and the anisotropically growing pattern, our series manipulator can perform stereotactic bioprinting due to the extra-DoF, which allows the print-head to adjust the printing direction according to the topography of the skin, making it feasible for skin with complex features, such as large inclination or downward surfaces.
Furthermore, compared with the traditional three-axis motion system, the robot provided a larger operation space with a smaller instrumentation volume. Figure 6 represents the illustrations and comparison of the convention 3D bioprinter and robot bioprinter in a clinical setting. In order to make the workspace cover a patient with 1.8 m in height, the occupied space of a three-axis bioprinter is around 2.66 m 3 , while the robot only takes 0.042 m 3 , which highlights the superiority of the later in portability. Moreover, the use of robot could avoid the risk of contamination. This advantage reflects the potential of the new 3D printing modality for clinical scenarios including X-ray exposure or pandemics infection , which can reduce the risk, and better protect the medical staff.
The application of bioprinting techniques to fabricate skin tissue substitutes for wound healing has been explored in previous studies, but rare of them achieved functional skin repair and far away from mature products of skin substitute due to the lack of skin appendages.
The replication of the native skin in morphological and physiological properties needs the incorporation of additional cell types and patterning of representative ECM components, and cannot be achieved simply rely on mimic architectural facets of living tissues. 67 In terms of cell type, as mentioned above, keratinocytes and fibroblasts were mostly used as seed cells for their extensive resources. Different printing processes including inkjet-based bioprinting, 9 extrusion-based bioprinting 23 and laser-assisted bioprinting 21 were applied to print a bilayered skin construct in vitro or directly into the defect site (in situ bioprinting). Most results showed high viabilities of printed cells after bioprinting as well as collagen secretion of fibroblasts and K14 expression of keratinocytes.
While the drawback was also prominent that little changes were shown in subsequent culture due to the low secretion activity and proliferation rate, which might be the main obstacle for further clinical application. High proliferation rate and functional activities of seed cells may be the complementary key to tissue construction and regeneration in vivo, which makes stem cells a favorable choice, including induced pluripotent stem cells (iPSCs), 68 mesenchymal stem cells, 24 and AFSCs. Though favorable wound closure has been achieved in these studies, the essential appendages in skin were still missing, and it inspired us to search for ideas from the research of hair follicle construction.
Previous studies involving hair follicle construction demonstrate that there are multipotent stem cells with the capacity to regenerate hair follicles and sebaceous glands in adult mammalian skin and this multipotency can be maintained in cell culture. 69 The hairdifferentiation potentiality of Epi-SCs can be activated by SKPs, and co-grafting of those cellular components from mice allows complete hair reconstitution, 70 which has also been proved in our previous study. 71 Compared with the cells used in most existing studies, these primary stem cells are promising for seed cells in 3D bioprinting skin constructs for their good proliferation, biological activities as well as the capacity of directional differentiation into fully functional skin.
However, most of de novo hair regeneration was observed in rodents, while demonstrating these regenerative properties in human cells and tissues has been challenging for human DP cells immediately lose their hair inductive potential after in vitro culture. Jahoda et al. 72 proposed that the striking difference between the behavior of human DP cells and those of mice is their propensity and capacity to aggregate both in vitro and in vivo. Based on this, Higgins et al. 73  The growth support of Matrigel on DP and sweat glands has also been verified in previous study. 77,78 The interaction of skin-derived stem cells with Matrigel acts in concert to give rise to a series of spatially and temporally coordinated events that regulate the stem-cells fate, specifically, directional differentiation into epidermis and dermis with necessary appendages.
Although the cytocompatibility of Matrigel is favorable, the clinical promise is limited due to its tumor-derived, ill-defined and variable composition. 79 The potential for antigenicity is one of the inherent limitations. As it is animal-derived, Matrigel might contain xenogenic contaminants, and the presence of growth factors and other biological proteins can lead to undesirable cellular effects, which limits its further application in other animal studies and even clinical trials. In addition, the mechanical properties of Matrigel show variability between batches, and it leads to an uncontrollable behavior when standard pneumatic-driven dispensing system applied, which needs additional procedures to regulate the printing process, such as volumetric-based extrusion and quick-response temperature control. 80 Even so, Matrigel can only provide the ECM-similar mechanical properties but cannot immediately present the apparent strength of skin tissues including tensile and compression modulus in kilopascal scale. 81 Thus, there might be potential risks for impairment in the structural integrity at the initial stage after bioprinting due to the scratching of mice, which also requires further improvement in the future. Herein, the F I G U R E 6 Comparison between conventional bioprinter and robotic bioprinter. *140 is calculated as double 70 in the symmetrical direction. **Maximum 1 layer means that the bioink cloud adheres to the vertical surfaces by increasing the viscosity of the material; however, the print head will still interfere with the printed structure Matrigel was used mainly for its favorable support for cell proliferation and migration. For future clinical translation of this strategy, synthetic biomaterials with highly biocompatibility, stable mechanical and apathogenic properties should be developed to provide appropriate alternatives to Matrigel. This research reports the development of an adaptive bioprinting robot, based on which, we performed a direct locationspecific printing with Epi-SCs, SKPs extracted from neonatal mice and Matrigel as bioink on a murine full-thickness excisional wound model. The evaluation of printability, efficiency, and therapeutic effect indicated that this method is a promising strategy for skin wound repair with the potential for clinical translation. But before that, these aspects need more attention and to be further studied:

| CONCLUSION
This study describes the proof-of-concept demonstration of an adaptive bioprinting robot to provide rapid on-site bioprinting using Epi-SCs, SKPs, and Matrigel as bioink on a murine full-thickness excisional wound model. The 3D scanning and closed-loop visual system integrated in the robot and the multi-DoF mechanism provide immediate, precise, and complete wound coverage through stereotactic bioprinting, which are important for maintaining homeostasis, wound closure, epithelialization, and scar prevention.
Matrigel provides a biomimetic ECM that serves as structural support for skin-derived stem cells, and stimulates the latter to differentiate into mature skin tissue that closely matches native skin, complete with epidermis, dermis, blood vessels, hair follicles and sebaceous glands etc. This study provides an effective strategy for skin repair through the combination of the novel robot and a bioactive bioink, and has a promising clinical translational potential for further applications.

CONFLICT OF INTEREST
The authors declare that they have no conflict of interest.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.